In recent years, there has been a rapid advancement in the development of potable and cordless electronic devices. With this development, the commercialization of non-aqueous electrolyte secondary batteries having a high voltage and a high energy density as the power sources for driving these electronic devices is proceeding.
The positive electrode for non-aqueous electrolyte secondary batteries usually contains a composite lithium oxide having a high oxidation-reduction potential. The most commonly used composite lithium oxides are lithium cobalt oxide, lithium nickel oxide and lithium manganese oxide. The negative electrode for non-aqueous electrolyte secondary batteries usually contains a carbon material. Non-aqueous electrolyte secondary batteries also contain a non-aqueous electrolyte prepared by dissolving a lithium salt in a non-aqueous solvent. As the lithium salt, LiClO4 and LiPF6 are typically used. Between the positive electrode and the negative electrode is disposed a separator. The separator is usually a microporous film made of a polyolefin resin material.
In the event where a short circuit of a relatively low resistance is caused inside a battery by some kind of factor, a large current flows intensively through the shorted point. The battery is thus heated and may reach a excessively high temperature. In order to prevent such phenomenon, various precautions are taken to provide safe batteries.
In the production aspect, the control of metal powders and the control of dust in the production atmosphere are conducted to prevent the intrusion of foreign matter into a battery. Another way is to protect the exposed portion(s) of current collectors having low resistance with, for example, an insulating tape so as to minimize the risk of an internal short-circuit.
Separators having shut-down function are also used. In the event were a short-circuit of a relatively low resistance occurs inside a battery, the pores of a separator having shut-down function close at about 135° C. so as to cut off an ion current. The short-circuit current is thus cut off, and heat generation stops. The surface temperature of the battery, however, increases to about 120° C.
In order to prevent an internal short-circuit, for example, Japanese Laid-Open Patent Publication No. Hei 7-220759 proposes to form, on an electrode, a 0.1 to 200 μm thick layer composed of an inorganic particle and a resin binder. An internal short-circuit often results from partial separation of a material from an electrode during the production of the battery. The above-mentioned publication is intended to improve the production yield by preventing such internal short-circuit.
Japanese Laid-Open Patent Publication No. Hei 9-208736 proposes to apply a heat-resistant resin (e.g., aramid) to a separator. This publication is also intended to prevent an internal short-circuit.
According to the above proposals, it is possible to reduce heat generation to a certain extent in the event where an internal short-circuit occurs locally. However, in nail penetration test which is a test to assess safety by simulating possible multiple simultaneous internal short-circuits that can cause damage to a battery, multiple shorted points occur simultaneously. Under such severe short-circuit conditions, the batteries disclosed by the above publications cannot always reduce heat generation, and the batteries may reach an excessively high temperature.
When a typical lithium ion battery, which comprises a positive electrode containing lithium cobalt oxide, a negative electrode containing graphite and a separator made of a polyethylene microporous film, is subjected to nail penetration test, the battery temperature increases until the separator exerts its shutdown function, and the surface temperature of the battery reaches about 120° C. This temperature increase is due to Joule heat generated inside the battery by short-circuit current.
Because the separator's shutdown function cuts the short-circuit current, heat generation is reduced before the battery temperature reaches more than 120° C. The safety evaluation standards for nail penetration test and crush test established by Japan Storage Battery Association require that batteries should be free from smoke, ignition and rupture, and no standard is set as to battery temperature. As such, even if the surface temperature of a battery reaches about 120° C., as long as the shutdown function works and heat generation is reduced, the battery is deemed to be satisfying the safety standards.
However, even if the safety standards are satisfied, when the surface temperature of a battery increases to about 120° C., the temperature of the electronic device containing the battery also increases, which may cause deformation of the body of the electronic device and impair safety of the electronic device. Under the circumstances, there is a desire for batteries with further enhanced safety and reliability. More specifically, there is a very strong desire to reduce the maximum battery surface temperature to 80° C. or lower even when an internal short-circuit occurs.
In the case of the battery disclosed by Japanese Laid-Open Patent Publication No. Hei 7-220759 having a 0.1 to 200 μm thick layer composed of an inorganic particle and a resin binder formed on an electrode, the battery surface temperature can reach as high as 80° C. or higher in the nail penetration test.
Similarly, in the case of the battery disclosed by Japanese Laid-Open Patent Publication No. Hei 9-208736 having a separator with a heat-resistant resin applied thereon, the battery surface temperature can reach as high as 80° C. or higher in the nail penetration test.
Therefore, according to the above-mentioned publications, battery surface temperature cannot always be reduced to not greater than 80° C. when multiple shorted points occur simultaneously. The reason that the battery surface temperature increases to higher than 80° C. in the nail penetration test can be explained as follows.
In the case where internal short-circuit occurs discretely, the layer composed of an inorganic particle and a resin binder as well as the heat-resistant resin prevent the shorted point from expanding. Because the shorted point burns out instantly due to its self heat-generation, the short circuit condition lasts only for 0.1 to 0.5 seconds. Thereafter, the electrical insulation recovers. Once the short-circuit current is cut off, the generated heat spreads out through the entire battery, and therefore the battery temperature does not increase to a high temperature. The rest (i.e., the area other than the shorted point) has a relatively low temperature, so that the heat spreads out smoothly.
In the case of the nail penetration test, in contrast, multiple shorted points occur simultaneously in a battery. Under such severe short-circuit conditions, heat is generated not only by the internal short-circuits, but also by thermal decomposition reaction of the positive electrode active material which generates heat continuously. Accordingly, the heat release rate at which heat spreads lags behind the heat generation rate, and thermal decomposition reaction of the positive electrode active material proceeds increasingly. This causes the separation or burn-out of the positive electrode active material near the shorted points. The positive electrode current collector (e.g., aluminum foil) is thus exposed to create another shorted point. As a result, such internal short-circuit condition is maintained, and the surface temperature of the battery increases until it reaches the temperature at which the separator exerts its shut-down function, namely, about 120° C. In the case of discrete internal short-circuit, the thermal decomposition reaction of the positive electrode active material does not proceed. As such, the separation or burn-out of the positive electrode active material does not occur, and thus additional shorted point is not created.